Lignocellulosic materials represent a vast amount of renewable resources available in virtually every part of the world. The use of lignocellulosics as a raw material for chemicals continues to be limited by the nature of current delignification processes (i.e., separation of lignin from cellulosic and hemicellulosic components) and by the difficulty of converting the lignin obtained to articles of commerce. This application is directed toward the latter. More particularly, this application is directed to the hydrogenolysis of a species of lignin to afford phenols, especially cresols.
Catalytic hydrogenolysis of lignin was known for some time to effect liquefaction, but its utility was severely curtailed by its tendency to afford a product of little commercial value. With the advent of the so-called Noguchi process (Canadian Pat. No. 700,210) it was claimed that a mixture of C.sub.6 -C.sub.9 monophenols would be obtained upon hydrogenolysis in yields as high as about 40%. The patentee used a catalyst of iron(II) sulfide with a co-catalyst of at least one sulfide of copper, silver, tin, cobalt, chromium, nickel, zinc, or molybdenum, and conducted the reaction in a solvent such as lignin tars and phenols at 250.degree.-450.degree. C. and an initial hydrogen pressure of 150-450 atmospheres. The process was extensively evaluated in a multitude of its variants, and although the high yields of monophenols as claimed by the patentee never could be reproduced the investigators concluded that the process remained the best one for lignin liquefaction to that date. David W. Goheen, Lignin Structure and Reactions, American Society, Advances in Chemistry Series, No. 59. However, another conclusion was that the process, even though the best one available, was economically unattractive because of the kind of lignin used, the relatively low economic value of the monophenol product mixture, and the loss of phenol itself when used as a solvent.
We have developed an improved and modified Noguchi process affording up to about 45% cresols and about 65% monophenols in the C.sub.6 -C.sub.9 range, thus substantially improving the economic return of lignin liquefaction. Additionally, we have developed a process where lignin liquefaction is effected in two stages which can be coupled so as to afford a continuous or semicontinuous process. Our improvements leading to the processes which are our inventions herein are based on several discrete but interrelated observations.
One observation is that when the catalyst composition is prepared in situ the sulfides formed are purer than if independently prepared, stored, and then used. A result is that our catalyst tends to give somewhat higher yields of monophenols. Another observation is that alkali lignin from the Kraft process may be used as the lignin source, thereby eliminating any steps necessary for the conversion of such lignin to another form prior to its use as a feedstock for liquefaction. An observation which is the cornerstone of our invention is that if methanol, as representative of lower aliphatic alcohols, is used in the reaction mixture the cresol yields are increased substantially. This is particularly important since cresols are perhaps the source of maximum economic return from the liquefaction of lignin. The final observation leading to our invention is that when lignin tar is used as a phenol substitute in a second stage reaction phenol is formed in sufficient quantity that it can be recycled to the first stage whose monophenolic products are chiefly cresols. This makes possible a process where an external source of phenol is used in initial liquefaction, but thereafter can be generated in sufficient quantity as to provide for the continued needs of the liquefaction process .